(1) Field of the Invention
The present invention relates to a liquid crystal display device, and particularly to a liquid crystal display device for which measures against display irregularities generated when semiconductor chips are mounted on a substrate are taken.
(2) Description of the Related Art
In a liquid crystal display device, provided are a TFT substrate on which pixel electrodes and thin-film transistors (TFTs) are formed in a matrix shape, and an opposed substrate which faces the TFT substrate and on which color filters are formed at positions corresponding to the pixel electrodes of the TFT substrate. In addition, liquid crystal is sandwiched between the TFT substrate and the opposed substrate. The transmittance of light by liquid crystal molecules is controlled for each pixel to form an image.
A liquid crystal display device is flat and light, and has been widely used in various fields, for example, a large-sized display device such as a TV, a middle-sized display device such as a monitor, and a small-sized display device such as a cellular phone or a DSC (Digital Still Camera). In particular, demand for liquid crystal display devices used in tablet liquid crystal modules has been increased.
In a liquid crystal display device, mounted are IC chips for driving scanning lines or video signal lines. In recent years, the IC chips are attached to the TFT substrate or the opposed substrate as a glass substrate by thermocompression bonding via anisotropic conductive films (ACPs). The temperature at this time is about 180° C. Such a method of mounting the IC chips is referred to as COG (Chip On Glass).
In the case of thermocompression bonding, the temperature of the IC chips differs from that of the glass substrate due to a difference between the heat capacity of the IC chips and that of the glass substrate. Thus, when the temperature is lowered, distortion of the glass substrate is generated. Further, the distortion of the glass substrate is generated after thermocompression bonding due to a difference between the coefficient of thermal expansion of the glass substrate and that of the IC chips. The coefficient of thermal expansion of glass is, for example, 3.8×10−6, and the coefficient of thermal expansion of the IC chips is 2.5×10−6. As described above, if the distortion of the glass substrate is generated, display irregularities are generated on a display area due to the effects of the distortion. Such display irregularities are referred to as COG irregularities in the description.
In order to suppress such distortion of the glass substrate, Japanese Patent Application Laid-Open No. 2008-20836 describes a configuration in which deformation suppression members in various shapes are attached between IC chips and an end portion of an opposed substrate. Metal or ceramics that is higher in rigidity than a glass substrate is used for such deformation suppression members.
In the technique described in Japanese Patent Application Laid-Open No. 2008-20836, rigid bodies are used to suppress the deformation of the glass substrate. However, since the rigid bodies are attached to the glass substrate using adhesive material, there is a possibility that distortion of glass is generated by the deformation suppression members depending on an attaching method or material. Further, the size and material of the rigid bodies are restricted in order to secure sufficient rigidity to suppress the deformation of glass.
FIG. 14 shows an example of irregularities to be eliminated by the present invention in a liquid crystal display device. In FIG. 14, a display area 11 surrounded by an, upper frame 10 is shown using white and COG irregularities 12 are shown by the hatching. However, the COG irregularities 12 are distinguished on a real black screen as shown by the hatching in FIG. 14. Specifically, the COG irregularities 12 are distinguished as black patterns on a black screen.
In FIG. 14, two IC chips 30 are mounted on each of a short side and a long side by the COG method on the upper side of the TFT substrate 100 or on the lower side of the opposed substrate 200 under the upper frame 10. The chips on the short side are those for driving scanning lines, and the chips on the long side are those for driving video signal lines.
FIG. 15 is a diagram obtained by removing the TFT substrate 100 and the opposed substrate 200 of FIG. 14. The TFT substrate 100 and the opposed substrate 200 adhere to each other through seal material (not shown). Liquid crystal is sandwiched between the TFT substrate 100 and the opposed substrate 200. Such a configuration is referred to as a liquid crystal display panel in the description. It should be noted that a polarizing plate is not shown in FIG. 15.
In FIG. 15, video signal line driving IC chips 30 are mounted at an end portion of the TFT substrate 100, and scanning line driving IC chips 30 are mounted at an end portion of the opposed substrate 200. These chips are connected to glass substrates by the COG method. The video signal line driving IC chips 30 are connected on the upper side of the TFT substrate 100, and the scanning line driving IC chips 30 are connected on the lower side of the opposed substrate 200. The length, width, and thickness of each IC chip are, for example, 13 mm, 1.5 mm, and 0.35 mm, respectively. Further, an interval d between the IC chips in FIG. 15 is, for example, 12 mm.
FIG. 16 is a cross-sectional view taken along the line E-E of FIG. 15. In FIG. 16, the IC chips 30 are mounted via ACFs 35 by the COG method. Areas of the TFT substrate 100 where the IC chips 30 are mounted are deformed to be in an upward convex shape on the both sides of each IC chip 30 as shown in FIG. 16. As described above, the deformation of the TFT substrate 100 has an impact on the display area 11 to generate the COG irregularities 12 as shown on the long side of the display area 11 of FIG. 14.
FIG. 17 is a cross-sectional view taken along the line F-F of FIG. 15. In FIG. 17, the IC chips 30 are mounted via the ACFs 35 by the COG method. Areas of the opposed substrate 200 where the IC chips 30 are mounted are deformed to be in a downward convex shape on the both sides of each IC chip 30 as shown in FIG. 17. As described above, the deformation of the opposed substrate 200 has an impact on the display area 11 to generate the COG irregularities 12 as shown on the short side of the display area 11 of FIG. 14.
An object of the present invention is to suppress the COG irregularities 12 as shown in FIG. 14 in the liquid crystal display device in which the IC chips 30 are mounted by the COG method.